JPS5856539A - Fm modulator for semiconductor laser - Google Patents

Abstract

PURPOSE:To stabilize the optical carrier frequency, by inserting a Fabry-Pe rot etalon to a negative feedback circuit of an FM modulator using a semiconductor laser. CONSTITUTION:A bias DC is applied to a laser 4 from a terminal 5, a signal S1 and an output of a differential amplifier 6 of a negative feedback loop are inputted to a synthesis circuit 7, and the output is inputted to the laser 4 to perform FM modulation corresponding to the change in the signal S1. A part of the output of the laser 4 is collected at a lens 8 and a Fabry-Pe rot etalon FPE11 is inserted into an optical path 9. The intensity of light is changed according to the frequency fluctuation of the FM signal changed at the FPE11 and the transmitted light is detected at a photodetector 13 via a lens 12. The optical signal of an optical path 10 is detected at a photodetector 16 via a lens 15. The output of the detectors 13 and 16 is inputted to a differential amplifier 14 to input a signal S2 proportional to the difference of the optical paths and an input signal S'1 having the equal amplitude with the signal S2 to the amplifier 6, then the output of the amplifier 6 is zero. If the frequency of the laser 4 is changed due to temperature change, the amplifier 6 generates an output cancelling the frequency change to stabilize the laser frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM modulator using a semiconductor laser. To explain further, this is a semiconductor laser FM modulator in which a Fabry-Perot Edalon is inserted into the negative feedback circuit of the modulation circuit to stabilize the optical carrier frequency.

Conventionally, PM variation l1Il of a semiconductor laser is performed using a subcarrier (8, C)8 whose frequency is several orders of magnitude lower than the frequency of light. Figure 1 shows a diagram of a conventional FMle@Manshiki program. The modulated am number is converted into jt (number 1) by a semiconductor laser (hereinafter referred to as laser) 4.

In the above FM modulator, since a subcarrier several orders of magnitude lower than that of the optical signal is used, there are disadvantages when the modulation band is wide,
Since a subcarrier is required, there are drawbacks such as a complicated circuit configuration.

In order to solve the above-mentioned difficulties, the present invention stabilizes the carrier wave and performs FM modulation directly with the input signal.
A novel semiconductor laser FM converter is provided in which the subcarrier is removed from the conventional modulation method.

For this purpose, the optical output section of the semiconductor laser is divided into two parts, a Fabry-Perot etalon is inserted into one optical path, and the optical output of both optical paths is divided into two optical paths.
The Z photodetector receives the light, and the respective outputs of the first photodetector and the second photodetector are outputted by the corresponding Igl differential amplifier Ili! The output of the first differential amplifier and the input signal are input to the second differential amplifier, and the output of the amplifier is inputted together with the input signal to the laser to which the DC bias signal is applied. This is the FM copper transformer, which uses a conductor that returns to stabilize the frequency.

The present invention will be explained below based on the drawings. In the lI2 diagram, the laser has a DC bias of 1! This shows the optical output when the current IJE-c flows.At the bias voltage K11, the laser enters the oscillation state and oscillates an optical signal of %7C. Under these conditions, when the signal fm is applied to the bias point (), the optical signal fc is subjected to FM modulation by the signal fs. In this case, the original signal fc is the temperature.

Varies due to fluctuations in power supply. When the light extinction 1c (=carrier wave) changes in FM change 11J, A change tJ4p
It can't be done.

As a solution to this problem, a negative feedback circuit is formed in the laser oscillation section to suppress signal fluctuations due to temperature and power fluctuations. ◎ For this purpose, the original signal of the negative coral ffi circuit is processed into a Fabry-Perot etalon (parallel). (a plate made of glass, etc. with a uniform refractive index) is also known as a plate of glass, etc. with a uniform refractive index.Thus, the frequency change is equal to the intensity change
Convert this.

Figure 3 (al shows the configuration of the 7-piece briebe opening - Edalon 11. It is made of a glass plate etc. with a plane parallel plate plate finish, and when the refractive index is n%JIlzadl, the wavelength λ (lambda) The element number 1M passes through the T core.The phase delay δ in the Fapley Bellow etalon ll is δ=4+/211nd.In this case, the transmission of the original signal fc is as shown in FIG. 3(b).

The fbJ diagram shows the intensity of transmitted light (vertical axis) with respect to λ (horizontal axis). If the thickness of Peronigeron tn is selected, the intensity of light changes in proportion to the change in wavelength λ (frequency).

FIG. 4 shows an embodiment of the present invention. In the blue, a direct current bias DC as shown in Fig. 2 is applied from the laser 4Ic terminal 5.
In this state, the signal 81 and the output of the second differential amplifier 6 of the east feedback loop are input to the synthesis circuit 7, and the output is input to the laser 4. Modulation is performed.

Here, in order to stabilize the oscillation output (optical signal fC) of the laser 4, the following negative feedback circuit is configured. That is, a part of the output of the laser 4 is focused by the lens 8, and $1 light w19! Insert the Faply bellows etalon 11. As a result, the intensity of the FM signal changes according to the frequency fluctuation by the Fapley Bellow etalon 11. This transmitted light is detected by the first photodetector 13 via the lens 12, and the detected signal is input to the inverting input terminal of the first differential amplifier 4914. The original signal of 21310 is detected by the third detector 16 via the lens 15, and the detected signal is input to the non-inverting input terminal of the differential amplifier 14.

An output proportional to the difference between the optical signal of the first optical path 9 and the optical signal of the @2 optical path IO is input to the non-inverting input terminal of the second differential amplifier 6, and the other fL@ input terminal 5 of the amplifier 6 is Input signal S
1' is input.

Here, when there are no temperature fluctuations or power supply fluctuations, the intensity of the laser light that passes through the Liebel etalon 11 and enters the first party detector 13 changes according to the frequency and amplitude of the signal S1. On the other hand, the intensity of the output of the second photodetector 16 changes according to the amplitude of the signal Sl. Therefore, the output of the eleventh second optical output device 13 and 14 is converted into the first differential amplification it! 1i14, only the intensity change corresponding to the frequency component of the signal 8 is output from the first differential amplifier 14. If the output S of the first differential amplifier 14 and the signal S, / are made equal in amplitude by 8, no output can be obtained from the second differential amplifier 6.

As is clear from FIG. 3(b), when the oscillation frequency of the laser 4 increases by Δf due to changes in temperature, power supply, etc., the intensity of light increases due to the Fabry-Bello ethane 11, and If the output is large, the j13 optical path, 20 is the fourth optical path, 21.22 is the lens, 23 is the 3rd optical detector, 2
4 indicates the 4th party detector, 25 indicates @3 differential width amplifier @ 11 @

Claims (1)

[Claims] Part of the optical output of the optical waveguide laser is divided into two and input into both optical paths, a Fapley bellow etalon is inserted into one optical path, and the optical output of both optical paths is divided into two and inputted into both optical paths. , the second party detector, the outputs of the 111 party detector and @2 optical detector are input to the positive terminal and inverting terminal of the corresponding Nl differential amplifier, and the output of the 11xl differential amplifier is 1m of the keeper power signal is input to the positive terminal (or positive terminal) of the second differential amplifier, and the output of the second differential amplifier is input to the positive terminal (or positive terminal) of the second differential amplifier. and the input signal are input to a synthesis circuit, and an output of the skeleton synthesis circuit is input to the semiconductor laser.